Compatibility of switch-mode LED drivers with conventional TRIAC-based leading-edge AC dimmer circuits has been a continuous challenge in solid-state lighting. Prior art came out with various ways to control the LED current magnitude in proportion with the output voltage or the conduction angle of an AC dimmer. However, it is typically desired in lighting to preserve the color correlated temperature (CCT), or the “white point” stability. Varying the LED current causes a shift in its chromaticity.
Alternatively, pulse-width modulation of the LED current can be used, while preserving the current magnitude, or so-called PWM dimming method. It is relatively easy to generate the necessary PWM control signal from the output waveform of the AC dimmer. Nevertheless, while improving the COT characteristic, this dimming method complicates the circuit implementation. Most switch-mode power supplies include a significantly large output filter capacitor. Hence, the LED load needs to be disconnected from the driver output using a load switch to obtain reasonable rising and fallingtimes of the LED current. Moreover, the PWM control signal needs to be communicated across the primary-to-secondary isolation barrier of a power transformer by using an optically isolated load switch gate driver circuit. Yet another problem arises from the finite response time of the constant-current feedback of the LED driver limiting the rise time of the LED current and potentially causing it to over-shoot.
Therefore, a need existed to provide a system and method to overcome the above problems.